This invention relates in general to extrusion-grade vinylidene chloride copolymer compositions and, additionally, to films prepared therefrom.
While many different materials, e.g., waxed papers, plastic coated papers, aluminum foil, and polyethylene film are used as protective wrapping, vinylidene chloride copolymer films are most widely used because of their low water and gas vapor transmission properties and resistance to mechanical and chemical deterioration as well as their adaptability to microwave cooking. In many instances, however, the vinylidene chloride copolymer film does not have sufficient cling and cohesion properties to adhere to itself when a small amount of pressure is applied as in household applications and, therefore, often does not adequately seal-in flavor, moisture, and other desirable properties of fresh or prepared food items.
Accordingly, various additives are frequently incorporated into the vinylidene chloride copolymer compositions from which film is made in order to increase the cling properties of the resulting film. In the case of extrusion grade vinylidene chloride copolymer compositions, for example, such additives are frequently in the form of liquids, such as citric acid esters, which typically perform the dual function of a cling enhancer as well as a plasticizer or extrusion aid.
Although the aforementioned additives have been useful in imparting cling properties to vinylidene chloride copolymer films containing them, such additives have introduced new problems. For example, the liquid cling additives have limited solubility in the vinylidene chloride copolymer resin and will sometimes exude to the surfaces of the film and excessively plate-out on or transfer to contacting members of film-making or film-handling machinery, thereby causing irregularities in operation or in the quality of the product.
Still further, such additives are incapable of providing high-cling properties while maintaining a balance of other physical properties. Although high-cling properties, which are required in films used for industrial application, can be achieved with the additives of the prior art by increasing the amount thereof, such increased cling is attended by a loss of good handleability. Either the films cling too much to themselves and present severe problems in handling because they cannot readily be pulled apart, (i.e., "a blocking" effect), or they are readily handleable but exhibit poor cling properties. In addition to the handleability problems effected by increasing the amount of such additives, other deleterious properties, such as increased permeability and cost, as well as decreased modulus of the film, tend to occur.
Accordingly, it is desirable to provide an extrusion grade vinylidene chloride copolymer composition capable of producing a film having inherent cling properties sufficient to enable it to form a seal upon the application of a small amount of pressure, yet which seal can be readily pulled apart. Furthermore, it would be desirable to provide such a high-cling/low-blocking film wherein the aforementioned problems of high permeability and cost, and decreased modulus are not as pronounced as in the past. It is also advantageous to provide such a composition having low vinylidene chloride and vinyl chloride residuals and low extractability of additives in use, which is easy to extrude and relatively inexpensive, has high-cling and is relatively stiff. This invention provides such a novel composition.